A product is typically wrapped in a heat shrinkable film by a process which results in forming an end seam by the application of heat transversely between sequential products. The film is then generally shrunk. The film around each wrapped product has a leading transverse end seam and a trailing transverse end seam. The shrinking is usually done by the application to the film of a heated fluid, typically a gas such as air or steam or a liquid such as water. This shrinking step is mainly undertaken in order to improve the appearance of the final package by the removal of wrinkles in the film.
A particularly important class of products for which film shrinking is needed is that of packaged food products, especially poultry parts. It is common to package poultry parts by (i) placing the poultry parts in a molded tray having a stiffening flange around its upper periphery and (ii) wrapping the tray and its contents in a heat shrinkable film. The film is then shrunk. There are certain problems inherent in packaging poultry parts in film which make the shrinking step particularly demanding in order to achieve a relatively smooth appearance. One of the problems is that the transverse end seams formed in the wrapping process typically extend outwardly beyond the tray flange after being sealed; it is desired to cause the sealed end seam to be hidden under the tray flange after shrinking. Another problem is that in cases where the height of the poultry product is greater than twice the height of the flange of the tray, the formed transverse end seams are higher than the flange. This situation makes shrinking the end seams so as to draw each end seam into a position under the flange especially difficult. Furthermore, the seam end portions at each package corner are frequently left extended after the film is shrunk so as to be visible beyond the package contours.
Government regulations require the marking of food storage and preparation information on the film. This marking is generally best done on the portion of film which is to be in contact with the bottom of the tray. If this film portion is shrunk by the direct application of heat, the printing frequently becomes distorted and difficult to read, thus not meeting the legibility standards set by the regulations.
Among the numerous prior patents which address the process of film shrinking are U.S. Pat. No. 5,193,290; U.S. Pat. No. 5,398,427; U.S. Pat. No. 5,546,677; and U.S. Pat. No. 5,787,682. The '290 and '427 patents disclose general film shrinking inventions and provide useful background. The '677 patent is directed to causing heat to be applied to the transverse seams of the film without directly heating the bottom film portion. The '677 patent discloses an invention in which the tray is turned 90° to travel “sideways” on its conveyor to facilitate end seam shrinkage. The '682 patent discloses an invention in which the tray is not turned sideways and moves continuously along a conveyor, with nozzles located below the conveyor. The nozzles are arranged to travel back and forth below the conveyor and their travel is timed such that the heated fluid from the first nozzle heats only the leading seam and the heated fluid from the second nozzle heats only the trailing seam. This arrangement, though effective, tends to be costly to manufacture and operate due to the need to move the nozzles during the process.
It would be desirable to provide a machine that does not require the packages to be turned sideways, that effectively shrinks portions of film at the end seams and that can be produced and operated in a more cost effective manner.